In any chromogenic photographic material it is desirable that the dyes formed should have certain properties. For instance the dyes should be bright in color, absorbing light in the appropriate spectral region, with very little secondary absorption so that good color reproducibility is obtained. It is also paramount that the formed photographic images should be resistant towards fading due to heat, humidity and light. When the dye images are formed in silver halide photographic materials from the combination of oxidized developer and an incorporated coupler, certain severe restrictions are placed on the properties of the coupler. For instance, the coupler should produce a dye which has the aforementioned desirable priorities. Also the coupler itself must show high efficiency in the dye-forming reaction, must be easily dispersible, must itself be resistant towards the deleterious effects of light, heat and humidity and must have a low propensity to form fog. It is well known in the art of coupler chemistry that when a functionality is incorporated into a molecule to achieve one of the aforementioned desirable properties (such as high dye light stability), quite often one or more of the other desirable properties of the photographically formed dye (such as its hue) is affected adversely. It is very difficult to obtain a coupler which manifests all or even most of the aforementioned desirable properties. There is still, for example, a great need for cyan dye-forming couplers which give rise to dye possessing exceptional stability against the deleterious effects of exposure to heat and humidity, but which at the same time retain a satisfactory level of the aforementioned desirable properties.
Cyan dye-forming couplers of the general structure described in this invention are well known in the art of photography, and it is also well known that image dyes derived from them exhibit excellent resistance to fading by heat and humidity. However, they are deficient in their ability to withstand the effects of light and their absorption bands tend to lie at a shorter wavelength than is desirable, particularly for color paper applications.
It is well know that the absorption characteristics of image dyes can be manipulated by incorporating certain functionalities into the molecular structure and that the chemical environment in which the dye is situated can also influence the hue of the dye. For example, U.S. Pat. No. 5,376,519 and JP 59171953 teach the use of certain phenolic coupler solvents to shift the dye absorption band to longer wavelengths. A disadvantage of the use of these phenolic coupler solvents is that the resultant image is prone to a thermally-induced post-process density enhancement, resulting from morphological changes to the image structure which increase the "covering power" of the dye.
The use of certain cyclic bisphenol phosphate or phosphonate esters to improve the stability of cyan image dyes is taught in U.S. Pat. No. 4,749,645 and JP 02008839. The U.S. Patent does not suggest the advantages of employing a particular phenolic solvent together with a particular type of cyan coupler.
A problem to be solved is to provide a photographic element comprising a cyan dye-forming formulation which yields a cyan image of good hue, and exhibiting exceptional stability against the effects of heat, humidity and light without significant degradation of other photographic properties.